Thanks for all suggestions, especially from Ikegami which led me to write the code under. There is also the possibility of using builtin caller(), but supporting these four ways to call seems more than enough flexibility and there is also the consideration of performance given that the particular method I am dealing with recurses millions of times for large inputs.

sub method {
        my $self = $_[0]; # leave it alone for obj/hashref
        unless( ref( $self ) || !$#_ ) { # else is immed. hash or
                                 # list of pairs
                my %opt = @_;
                $self = \%opt;
        }

        # now $self can be processed as a hashref whichever
        # of the four calling styles was used
}
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